Bluetongue virus (BTV) (Reoviridae family, Orbivirus genus) is a non-enveloped, icosahedral double-stranded RNA virus. Several protein layers enclose the genome. Upon cell entry the outer layer is stripped away leaving a core whose surface is composed of VP7. The structure of the trimeric VP7 molecule has been determined using X-ray crystallography. The articulated VP7 subunit consists of two domains, one largely alpha-helical and the other a smaller domain with the familiar 'jelly-roll' topology. The relative orientations of these two domains vary in different crystal forms. A 23 resolution map of the core determined, using electron cryo microscopy (cryoEM) data, reveals 260 trimers of VP7 organized on a rather precise T=13 laevo icosahedral lattice. The VP7 layer occupies a shell between 260[unreadable] and 345[unreadable] from the center of the core. Below this radius (230-260[unreadable]) lies the T=1 layer of 120 molecules of VP3. By fitting the X-ray structure of the individual VP7 trimer to the cryoEM structure, we have generated an atomic model of the VP7 layer of BTV and demonstrated that one of the molecular structures seen in crystals of the isolated VP7 corresponds to the conformation of the molecule in the core. The beta barrel domains of VP7 are external to the core and interact with the protein in the outer layer of the mature virion. The lower alpha-helical domains of VP7 interact with the inner layer of VP3. Adjacent VP7 trimers in the T=13 layer interact principally through well defined regions in the broader lower domains and conform well to the structure expected from the theory of quasi-equivalence, with no significant conformational changes within the individual trimers. The VP3 layer determines the particle size and forms a rather smooth substrate upon which the two dimensional lattice of VP7 trimers is laid down.